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Full text of "Directions for industrial use of aerosols"

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March 1952 E-835 

United States Department of Agriculture 

Agricultural Research Administration 

Bureau of Entomology and Plant Quarantine 


By A. H. Yeomans 
Division of Stored Product Insect Investigations 

Aerosols are used in many industrial structures to control insects 
by direct contact or by applying a light deposit of insecticide on the top 
of exposed horizontal surfaces. 

An aerosol is composed of a number of fine liquid particles suspended 
in the air. It is considered that in an insecticidal aerosol all the particles 
should be smaller than 50 microns in diameter and 80 percent by weight 
of them should be less than 30 microns. Vapors or smokes composed 
of particles of smaller diameter than 0.1 micron are not classed as 
insecticidal aerosols because they exhibit slightly different character- 

Aerosols may be produced by liquefied-gas formulations released 
through capillary or expansion chamber nozzles, by steam or air 
atomization of liquid, by spinning disks and rotors, by forcing liquid 
under high pressure through atomizing nozzles, by heat vaporization, 
or by a combination of these methods. 

A wide range of particle sizes can be produced, and the size of the 
particles has a great influence on the effectiveness of the aerosol. The 
particle size determines the time the aerosol remains suspended in the 
air and therefore the amount of dispersion by air currents throughout 
the enclosure. The particle size is a critical factor influencing the 
amount that collects on an insect as it flies through the aerosol. If the 
particles are too small, they are deflected from the flying insect as 
smoke is from a moving automobile. If they are too large, they settle 
rapidly and their dispersion is poor; therefore their chance of contacting 
the insect is also poor. When an insect does collide with an oversized 
droplet, the excess insecticide is wasted. 

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Settling Rate and Dispersion 

Aerosol particles tend to settle vertically at a rate related to their 
size. The time required for oil particles to settle 10 feet is given 

in microns 


in microns 


1 26 1/2 hours 

5 72 minutes 

10 19 

15 15 

20 5 minutes 

30 2 

50 45 seconds 

100 11 

Water droplets settle slightly faster. The lateral dispersion of 
aerosols is accomplished by air currents after the small impetus from 
the atomizer is expended. Aerosol particles will not be conveyed into 
dead-end cracks or into materials through which air does not circulate. 
As would be expected, small particles disperse laterally to greater 
distances than do larger ones as they are suspended in air for a longer 
time. In unheated buildings air currents are at a minimum, but 
heating sets up convection currents that are a great aid to dispersion. 
In some cases it is necessary to aid dispersion with large-volume air 
blowers. Table 1 shows the dispersion of aerosols of various particle 
sizes in an unheated room with a ceiling height of 8 feet. 

Table 1. --Lateral dispersion of aerosols of various particle 
sizes released at the ceiling level in an unheated room 
8 feet high 


rcent of particles of indicated 

Feet from 

mass median diameter 


5 microns 

15 microns 

25 microns 

45 microns 



66 84 




29 15 




4 Trace 

















About 95 percent of an aerosol settles on the top of horizontal 
surfaces, the remaining 5 percent deposits on the walls and ceiling. 
The amount of deposit on a horizontal surface depends on the concen- 
tration of the aerosol directly above that surface. Therefore, if the 
aerosol is evenly dispersed throughout a room, the deposit will be 
proportional to the distance of the surface from the ceiling. The deposit 
on walls and ceilings is mainly on small protrusions, such as fibers, 
on rough surfaces which catch the particles as they go by on air currents 
or by settling. Insects resting on the walls and ceilings will also be 
struck by an occasional particle as it passes by. Surfaces colder than 
the air temperature slightly attract very small particles. 

Selecting Particle Size 

No single particle-size range is suitable for all conditions where an 
aerosol may be used. The particle-size range should be selected after 
the factors involved in a proposed treatment have been evaluated. The 
most important factor is time. 

If the aerosol is to be applied to a structure or room that can be 
closed for several hours, small particles will give the best dispersion 
and penetration into small crevices. An aerosol with particles of about 
5 microns mass median diameter was applied in a large warehouse. 
A thermal generator was operated outside of the building, introducing 
the aerosol through an open door. The aerosol dispersed along the 
ceiling, and by the time the proper amount had been applied it was well 
distributed throughout the warehouse by convection currents. The 
warehouse was then closed overnight and the aerosol allowed to settle. 
Uniform distribution resulted. 

When the treatment must be limited to a short time, larger particles 
are necessary. A 10- to 15-minute exposure is sufficient with an aerosol 
having particles with a mass median diameter of 15 to 20 microns, 
as these particles will settle 10 feet within 5 to 15 minutes. Where 
flying insects are to be controlled, this type of treatment is quite satis- 
factory. However, since lateral dispersion is restricted when particle 
sizes of this range are used, in large rooms or structures, the aerosol 
must be released from several points in order to give uniform dispersion. 

Many types of application will fall between these two extremes. The 
proper relation between time of exposure and particle size can be cal- 
culated from the tabulation on page 2. The allowable settling time should 
be based on the smallest particles. The approximate distances for 
uniform dispersion can be determined from table 1. Better dispersion 
can be obtained when heating sets up convection currents or when fans 
are used. 

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3 1262 09239 6331 

After considering the foregoing factors, the operator can select the 
most desirable particle size to meet his need, and then the type of 
generator that will produce the desired particle-size range and volume 
of aerosol. If the available equipment will not produce the desired particle 
sizes, then the time will have to be adjusted to the particle-size range that 
can be produced. 

It has been demonstrated that particles above certain sizes, when com- 
posed of some solvents and insecticides used in aerosol formulations, will 
injure plant foliage. Persons treating greenhouse's with aerosols should 
keep this factor in mind when selecting the particle size. 

Some structures may be too open for the successfull use of aerosols 
because of too much loss through the wall openings. For instance, in 
large open warehouses full of tobacco hogsheads, where an aerosol could 
not be contained in the structures, it was found that a mist spray with 
particles of about 50 microns mass median diameter, blown over the top 
of the hogsheads by a mist blower, gave a uniform deposit of insecticide 
and good insect control. 

We have used the following formulations indoors: 

1 pound of technical DDT dissolved in 7 1/2 pints of Sovacide 544C 
(Socony Vacuum) to make 1 gallon. 

1 pound of technical DDT dissolved in 2 quarts of carbon tetra- 

chloride and then 3 1/2 pints of Texaco 300 oil added to make 
1 gallon. (This material is noxious to breathe but relatively 
safe from explosion.) 

2 quarts of a 10-percent pyrethrum in deodorized kerosene with 

1 quart of piperonyl butoxide and 1 quart of Texaco 300 oil added 
to make 1 gallon. 

Owing to the explosion hazard when oil solutions are used indoors, not 
more than 1 gallon of these solutions should be used per 100,000 cubic feet, 
and they should not be released in the vicinity of an open flame. All work 
indoors should be done while wearing a proper respirator. The pyrethrum 
formula is recommended for use around exposed foodstuffs. 

Some formulations contain a proportion of relatively nonvolatile oil 
to maintain the desired particle size while it is suspended in the air. 


The dosage will vary according to the insect and the insecticide used. 
Because of better dispersion, rooms with high ceilings require a smaller 
dosage than those with low ceilings. In closed warehouses there are 
indications that 1 pound of DDT in 1 gallon of solution per 6,500 square 
feet of floor space applied about every 2 weeks in summer will provide 
excellent protection of commodities against insect infestation.